Pneumatic transmission systems are widely known and are used to transmit articles from one place to a remote location. Pneumatic transmission systems usually include at least two stations, a tube or conduit extending between the two stations, and a carrier positioned within the tube so as to be delivered by pneumatic pressure. The pressure can be a superatmospheric pressure or a subatmospheric pressure.
A common use for a pneumatic transmission system is in drive-in bank teller facilities where business is conducted via a carrier transmitted between the bank and the remote drive-in terminal. Other uses include sending documents between different floors in a building, or from one office to another office located some distance apart.
An example of a conventional pneumatic transmission system that used a pair of blowers is shown in FIG. 1. A first station 30 and a second station 35 are connected by a transmission tube 40. A first blower 10 is located at the first station 30 and can pressurize the air behind a carrier 45, thereby creating enough differential pressure (.DELTA.P) across the carrier 45 to push the carrier 45 upwardly from station 30, then along the horizontal section of the tube 40 to the second station 35. Similarly, the second blower 20, which is located at the second station 35, can pressurize the air behind the carrier 45 and send the carrier 45 in the opposite direction toward the first station 30. In one such system, the blowers were a pair of vacuum cleaner motors which were physically and electrically isolated from each other so that each blower 10 and 20 was operated independently of the other blower. The first blower 10 can be turned on by actuating a first mechanical switch 15, sending a carrier 45 from the first station 30 to the second station 35. The second blower 20 can be turned on by actuating a second mechanical switch 25 to send a carrier 45 from the second station 35 to the first station 30. If a first carrier 45 was inserted in the first station 30 and the blower 10 was turned on and then a second carrier 45 was inserted in the second station 35 and the blower 20 was turned on while the first carrier 45 was in transit, thereby placing two carriers in the transmission tube 40 simultaneously, the movements of the two carriers 45 would be blocked until one of the blowers 10 or 20 was turned off, at which time both carriers would proceed in the direction dictated by the blower which remained on.
In many pneumatic transmission systems, the carrier would travel through the tube and impact a stop device once it had reached its intended destination. In such systems the carrier can travel at speeds of 15-20 feet/second or higher, and the impact of the carrier against the stop device can cause great wear on both the carrier and the system as well as damage the contents of the carrier.
One method for obviating the high velocity impact between the carrier and the stop device has employed the use of an air cushion adjacent to the receiving terminal, as illustrated in FIG. 2. The air cushion is created by pneumatically sealing the receiving terminal 50 (making it a closed terminal) and providing a vent 65 (or check valve) in the tube 75 a short distance from the receiving terminal 50 such that when the carrier 70 passes the vent 65 in an approach to the receiving terminal 50, a trapped column of air is created in the approach leg 55 of the tube 75 which serves to decelerate or "cushion" the carrier 70 as the carrier 70 makes its final approach to the receiving terminal 50. The check valve 65 is opened to the atmosphere either directly, or through a conduit 60 as shown. However, such an air cushion system requires that the receiving terminal have a door capable of pneumatically sealing the terminal. The system operator must then manually open the terminal door in order to retrieve the carrier from the system. Alternatively, a complicated mechanism can be provided to automatically open the terminal door upon the arrival of the carrier. However, such mechanisms are often costly and prone to mechanical failures at inopportune times.
This form of operation is well known in the art of pneumatic systems. However, slowing down a carrier is not this simple when the destination station is an open air station. There is no dead column of air when the station is open to the atmosphere because the air in front of the carrier is exhausted out of the open station. Therefore, there is no pressure build up in front of the carrier and there is no slowing force to act upon the carrier.
Other attempts to resolve the high impact problem have included the use of other trigger means to shut off the stream of air. These alternative trigger means include such items as a photocell, a timing device, a limit switch, a spring catch, and combinations thereof.
An alternative system in which a carrier is decelerated prior to entering an open terminal is disclosed in U.S. Pat. No. 4,180,354 to Greene. U.S. Pat. No. 4,180,354 discloses a transmission system in which the pressurized air behind the carrier is routed principally through a check valve positioned near the open terminal to allow the carrier sufficient time to decelerate before discharging into an open terminal. An adjustable valve allows some air to continue to push the carrier to the terminal. A secondary air line adjacent to the open terminal draws in the air from the main transmission line and reroutes it to the blower, thus avoiding the blowing of air through the open terminal. The carrier is decelerated by simply choking off most of the air behind it at a point near the open terminal so that the carrier ejects with a minimum speed from the transmission line into the open terminal. The above cited system is a way to slow a carrier as it approaches an open destination terminal. However, this system requires multiple routing conduits and an adjustable valve to achieve the desired result.
Still another alternative system in which a carrier is decelerated prior to entering an open terminal is disclosed in U.S. Pat. No. 4,984,939 to Foreman et al. This patent discloses the use of one pressure blower and one vacuum blower, wherein the vacuum blower is operated at an equal or greater capacity than the pressure blower. The pressure blower and vacuum blower are attached to the transmission conduit by air tubes at different locations along the transmission conduit. In this system, a carrier is sent from a first station to a second station by activating the pressure blower at a certain capacity to create a .DELTA.P across the carrier thereby moving it out of the first station, through the transmission conduit and toward the second station. The vacuum blower is attached to the transmission conduit at some point near the second station. As the carrier approaches the second station, the carrier is slowed by the counter flow of air due to the vacuum blower. The vacuum blower sucks air out of the transmission conduit behind the carrier at an equal or greater capacity than the pressure blower, which reverses the .DELTA.P across the carrier and slows the carrier as the carrier makes its final approach to the open terminal.
As mentioned above, this system requires multiple transmission conduits and precise timing in order to operate effectively.
Conventional pneumatic transmission systems are also used in multi-station configurations, such as in a hospital. In these systems, one central station, a laboratory for example, sends a carrier to any one of many receiving stations such as nurse stations. The cargo inside the carrier in these systems can be fragile. Therefore, it is advantageous to allow the carrier to enter the open stations at a low rate of speed in order to maintain the integrity of the cargo. Conventional multistation pneumatic transmission systems currently have to use a slide gate at each station in order to achieve this result. The slide gate is a combination of a door and a motor, which is activated as the carrier enters the particular station. Upon approach of the carrier, the motor is activated and the slide gate positions itself inside the transmission conduit. The slide gate effectively closes the transmission conduit in front of the carrier, thereby forming a dead column of air in front of the carrier. The carrier is slowed as it falls on the dead column of air, and finally comes to rest on the slide gate. The slide gate is then removed from the transmission conduit, allowing the carrier to drop into the open station.
The problems associated with the above described conventional multi-station pneumatic transmission systems include, among other things, the cost of numerous slide gates and the lack of reliability due to the use of additional moving parts.